The present invention relates to an avalanche photodiode (to be referred to as an APD hereinafter) and, more particularly, to an APD used as a photodetector for optical communication.
It is generally known that a noise/gain characteristic or a gain-bandwidth product of an APD mainly depends on a ratio between an electron ionization ratio .alpha. and a hole ionization ratio .beta. and these characteristics are improved as the ratio .alpha./.beta. or .beta./.alpha. is increased.
Since this ratio of an APD consisting of Ge or InP used in present optical communication systems is close to 1, its performance is limited in terms of high noise or the like. In order to increase this ratio, it is known that a superlattice obtained by alternately stacking two types of semiconductors can be effectively used. (R. CHIN et al., Electronics Letters, 18,487 (1980) and F. CAPASSO et al., Applied Physics, 40,38 (1982)). In particular, it was found that an ionization ratio of electrons was higher than that of holes by about 20 times in an InGaAs/InAlAs superlattice having large discontinuity in a conduction band, and a method of increasing the .alpha./.beta. ratio on the basis of this finding has been proposed. (T. Kagawa et al., Applied Physics Letters No. 55, PP. 993-995 (1989) and K. Brennan, IEEE Transaction, Electron Devices ED-33, 1502 (1986)).
The reported superlattice APD, however, had a structure in which a nondoped InGaAs/InAlAs superlattice was sandwiched between p.sup.+ -InGaAs and n.sup.+ -InGaAs having a high impurity concentration (about 1.times.10.sup.18 cm.sup.-2) or between similarly heavily doped p.sup.+ -InAlAs and n.sup.+ -InAlAs.
In the former structure in which the InGaAs/InAlAs superlattice is sandwiched between p.sup.+ -InGaAs and n.sup.+ -InGaAs, light incident from the p.sup.+ -InGaAs side is absorbed by p.sup.+ -InGaAs and pairs of electrons and holes are generated in this portion. The generated electrons enter the superlattice to cause multiplication. Since no electric field is applied to p.sup.+ -InGaAs, these electrons must reach the superlattice by diffusion. This degrades the RF characteristics of an element because the transit of electrons obtained by diffusion is very slow.
On the other hand, in the latter structure in which the InGaAs/InAlAs superlattice is sandwiched between p.sup.+ -InAlAs and n.sup.+ -InAlAs, incident light is transmitted through p.sup.+ -InAlAs and absorbed by the InGaAs/InAlAs superlattice. In this case, since an electric field is applied to the superlattice where electron-hole pairs are generated, no degradation is caused in RF characteristics by diffusion. However, since avalanche multiplication is caused by mixed injection of electrons and holes, the effect of the superlattice structure of improving the ionization ratio is canceled. That is, noise remains large, and a gain-bandwidth product is not improved. (When a multiplication layer in which the ionization ratio of electrons is higher than that of holes is used, neither a reduction in noise level nor an increase in gain-bandwidth product can be achieved unless multiplication of carriers is caused by selectively injecting only electrons into the multiplication layer.)
In addition, in conventional superlattice APDs, since InGaAs/InAlAs is used in a superlattice layer, a high electric field is applied to an InGaAs well layer having a small band gap to allow electrons to flow through this portion by a tunnel effect at a high probability. Therefore, an amount of a dark current is undesirably increased by the tunnel effect. Also, pileup of holes having a large effective mass is caused by discontinuity in a valence band to degrade RF characteristics. Furthermore, since the superlattice layer is not transparent to signal light, a back-surface incident structure cannot be formed.